In certain communication systems, data is sent by altering the characteristics of a wave form of a signal, which is generally known as a carrier wave, by some characteristics of a second signal that contains the data to be communicated. This process is known as modulation. Many different types of modulation methods are known. For example, one common modulation technique is amplitude modulation. In amplitude modulation (AM) the amplitude of the carrier wave varies in proportion to the data bearing signal. Another type of modulation is called frequency modulation (FM), which involves altering the instantaneous frequency of the carrier wave based on the amplitude of the data bearing signal. Both AM and FM are considered analog modulation methods since both the carrier wave and the data carrying wave are sinusoidal waves.
In digital communication systems, the carrier wave is modulated by a digital signal. Many different types of modulation techniques for digital communication exist. One type of modulation is phase shift keying modulation (PSK). Modulation of the phase of the carrier wave is varied to convey information. Another type of digital modulation is differential phase-shift-keying (DSPK), in which the modulating signal is not the binary code of the digital signal but a code that records changes in the binary code of the digital signal. Therefore, the demodulator only needs to determine changes in the incoming signal phase.
DPSK modulation is used to modulate data transmissions from ground stations to aircrafts using the Mode-S air traffic surveillance system. In a Mode-S system, a ground station sends out an “All Call” interrogation signal to aircraft in the ground station's coverage area. An aircraft with a Mode-S transponder receives the interrogation from the ground station, and sends a reply that includes a unique identifier of the aircraft. Once the ground station records the unique identifier, the ground station is able to directly communicate with the replying aircraft.
The DPSK encoded carrier wave is received by the aircraft radio and decoded by an on board demodulator. Typical aircraft demodulators are analog devices that include analog delay lines, phase detectors, amplified comparators and other components to demodulate the received carrier wave. The use of analog components increases the size, complexity, and cost of a demodulator.
In view of the foregoing, it is desirable to provide a digital DPSK demodulation method that addresses one or more of the foregoing deficiencies or other deficiencies not implicitly or expressly described. It is also desirable to provide a system for determining a digital DPSK demodulation that addresses one or more of the foregoing deficiencies or other deficiencies not implicitly or expressly described. Furthermore, other desirable factors and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.